Stage Line 500W amp repair project

[KrisBlueNZ]

Sorry, my mistake. The "N.F." marking on the schematic means "not fitted", so they're not supposed to be there. But could you look into Q7 please.

 

[KrisBlueNZ]

Also, can you measure three more voltages, all around Q7. Leave the link wire in place.

 

[bikemech]

here is the location of Q7, its not actually touching the heatsync its aproxamately 3mm away

 

[bikemech]

 

[bikemech]

is this the correct pinout for Q7??, i mesured is as being EBC looking at it from the front

 

[KrisBlueNZ]

Yes that's the right pinout for the BC547/8/9 and BC557/8/9. So the voltages you marked on the schematic in post #144 have the collector and emitter reversed. That's no problem.

Those voltages are reasonable. Is the left channel fault LED still lit?

 

[KrisBlueNZ]

Also, Q7 and the corresponding transistor in the right channel will need to be held in contact with the heatsink. Can you think of any way to do this? You can use a small amount of thermal paste to get heat conduction but you really need some way to hold them together with some pressure.

 

[bikemech]

I am sure I could find a way of securing them to the heatsink, is it necessary as they have never been that way from new, or would it improve efficiency that way?

 

[bikemech]

Yes that's the right pinout for the BC547/8/9 and BC557/8/9. So the voltages you marked on the schematic in post #144 have the collector and emitter reversed. That's no problem.

Those voltages are reasonable. Is the left channel fault LED still lit?

Is it just certain transistors reversed like this, I have done all my previous measurements the other way round, only reason I noticed this was different was because I wanted to know a little more about this one with it being heat sensitive, so I searched the part number and came across the pin out ,
And yes fault left LED is still lit.

 

[KrisBlueNZ]

Yes, you're absolutely right. The BC54x transistors use the European pinout, which is the mirror image of the American pinout that the MPSAxx transistors have. There's also a Japanese pinout, and a fourth pinout that's used for some transistors that operate at high frequecies (radio, etc).



That transistor is used to stabilise the operating current of the output stage and it needs to be in good contact with the heatsink. I'm surprised that it's not clamped to the heatsink; that's how it's normally done. Here's how it works.

The output transistors in an amplifier operate with a steady current flowing through them. This is called the quiescent or idle current. As the output transistors warm up, their characteristics change slightly, and if the driving circuitry kept the drive conditions the same, the output transistors would conduct more current. This would make them get hotter, which would make them conduct more current, and so on, until eventually they fail, and then the fuses blow. This is called thermal runaway.

That little transistor monitors the temperature of the output transistors, and when they get warmer, it gets warmer too, so its characteristics change slightly too, and this slightly reduces the bias voltage to the output transistors. This compensates for the changing characteristics of the output transistors, and keeps the quiescent current constant. This prevents thermal runaway.

I'm a bit concerned that this amplifier was apparently designed with those transistors just standing next to the heatsink. This will delay the response to temperature changes, which I don't think is a good thing, although the forced fan cooling will probably have some bearing on this. I think we should attach the transistors to the heatsink, and once it's up and running, do some testing to see how the quiescent current varies with temperature.

I'm looking into the protection circuit and I'll get back to you with some more tests.

 

[bikemech]

are we anywhere near to having this operating correctly?
I don't mind if we're not,
Also where some of the components are not fitted on the schematic, is it possible to fit some of them to make this amp better, or would it be pointless,
I have no idea wher the missing circuitry would have done.

 

[KrisBlueNZ]

It's hard to say how close we are. There might be just one problem to fix, then everything will go smoothly, or we might have to spend a while fault-finding.

There is a second set of output transistors that are not fitted, that would improve the reliability and drive capability of the amp. You can see the blank areas on the heatsinks and the board where they would go. But the manufacturer has not drilled holes for the base and emitter leads - either in the circuit board, or in the heatsinks. And they have used one of the mounting screws in each channel for the overtemperature detection transistor. If you want to move the transistor and drill the extra holes, I would say go ahead.

I'm not going to be able to do any more on this for the next few days so I wish you a Merry Christmas and I'll get back to you

 

[bikemech]

Same here!, merry Christmas and thanks for your help so far,

 

[bikemech]

Any joy yet kris, hope you had a good Xmas and new year.

 

[ike2903]

Good!Happy new year to all!
Hope all people: A good start.

 

[KrisBlueNZ]

Hi again! Happy New Year and sorry for the delay.

OK, I need some more voltages.

 

[bikemech]

hi kris, sorry for late reply been ill with flue, here is the results of Q46

 

[KrisBlueNZ]

I'm sorry to hear that. I hope you're feeling better now.

I need two more voltages.

 

[bikemech]

here is D38 zener readings

 

[bikemech]

If I could get my head around understanding the current flow of the schematic I could test the next component , and so on